From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id CB0B3C07E9D for ; Fri, 23 Sep 2022 23:02:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232630AbiIWXCo (ORCPT ); Fri, 23 Sep 2022 19:02:44 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48154 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229511AbiIWXCn (ORCPT ); Fri, 23 Sep 2022 19:02:43 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 09D69128A22 for ; Fri, 23 Sep 2022 16:02:42 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 84A3B61CE8 for ; Fri, 23 Sep 2022 23:02:41 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id A633FC433C1; Fri, 23 Sep 2022 23:02:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1663974161; bh=HZY9IBKYnlYGqpe1EtnMsuCj5J8tHQ7n1LBae9biGtU=; h=Date:To:From:Subject:From; b=HNXDY15JBGCSG9L+oVsBSbB6sasx9fAEXxc8xEneKNI3xiuF5gxExw2M6dczytPIJ trDU/cOQCziJsrKy9GG6xkbESsaygqwDnNBpP/iCE43S/XBRO1S1Mfdzc74nRd/lOa /HLeVQLDaZ756Oko47JyX3CTDxJVQwo6UchTBbUA= Date: Fri, 23 Sep 2022 16:02:39 -0700 To: mm-commits@vger.kernel.org, vgoyal@redhat.com, npiggin@gmail.com, mpe@ellerman.id.au, mingo@kernel.org, lukas.bulwahn@gmail.com, joe@perches.com, jani.nikula@linux.intel.com, dyoung@redhat.com, dwaipayanray1@gmail.com, David.Laight@ACULAB.COM, corbet@lwn.net, christophe.leroy@csgroup.eu, bhe@redhat.com, apw@canonical.com, david@redhat.com, akpm@linux-foundation.org From: Andrew Morton Subject: [to-be-updated] coding-stylerst-document-bug-and-warn-rules-do-not-crash-the-kernel.patch removed from -mm tree Message-Id: <20220923230240.A633FC433C1@smtp.kernel.org> Precedence: bulk Reply-To: linux-kernel@vger.kernel.org List-ID: X-Mailing-List: mm-commits@vger.kernel.org The quilt patch titled Subject: coding-style.rst: document BUG() and WARN() rules ("do not crash the kernel") has been removed from the -mm tree. Its filename was coding-stylerst-document-bug-and-warn-rules-do-not-crash-the-kernel.patch This patch was dropped because an updated version will be merged ------------------------------------------------------ From: David Hildenbrand Subject: coding-style.rst: document BUG() and WARN() rules ("do not crash the kernel") Date: Tue, 20 Sep 2022 14:23:00 +0200 Linus notes [1] that the introduction of new code that uses VM_BUG_ON() is just as bad as BUG_ON(), because it will crash the kernel on distributions that enable CONFIG_DEBUG_VM (like Fedora): VM_BUG_ON() has the exact same semantics as BUG_ON. It is literally no different, the only difference is "we can make the code smaller because these are less important". [2] This resulted in a more generic discussion about usage of BUG() and friends. While there might be corner cases that still deserve a BUG_ON(), most BUG_ON() cases should simply use WARN_ON_ONCE() and implement a recovery path if reasonable: The only possible case where BUG_ON can validly be used is "I have some fundamental data corruption and cannot possibly return an error". [2] As a very good approximation is the general rule: "absolutely no new BUG_ON() calls _ever_" [2] ... not even if something really shouldn't ever happen and is merely for documenting that an invariant always has to hold. However, there are sill exceptions where BUG_ON() may be used: If you have a "this is major internal corruption, there's no way we can continue", then BUG_ON() is appropriate. [3] There is only one good BUG_ON(): Now, that said, there is one very valid sub-form of BUG_ON(): BUILD_BUG_ON() is absolutely 100% fine. [2] While WARN will also crash the machine with panic_on_warn set, that's exactly to be expected: So we have two very different cases: the "virtual machine with good logging where a dead machine is fine" - use 'panic_on_warn'. And the actual real hardware with real drivers, running real loads by users. [4] The basic idea is that warnings will similarly get reported by users and be found during testing. However, in contrast to a BUG(), there is a way to actually influence the expected behavior (e.g., panic_on_warn) and to eventually keep the machine alive to extract some debug info. Ingo notes that not all WARN_ON_ONCE cases need recovery. If we don't ever expect this code to trigger in any case, recovery code is not really helpful. I'd prefer to keep all these warnings 'simple' - i.e. no attempted recovery & control flow, unless we ever expect these to trigger. [5] There have been different rules floating around that were never properly documented. Let's try to clarify. [1] https://lkml.kernel.org/r/CAHk-=wiEAH+ojSpAgx_Ep=NKPWHU8AdO3V56BXcCsU97oYJ1EA@mail.gmail.com [2] https://lore.kernel.org/r/CAHk-=wg40EAZofO16Eviaj7mfqDhZ2gVEbvfsMf6gYzspRjYvw@mail.gmail.com [2] https://lkml.kernel.org/r/CAHk-=wit-DmhMfQErY29JSPjFgebx_Ld+pnerc4J2Ag990WwAA@mail.gmail.com [4] https://lore.kernel.org/r/CAHk-=wgF7K2gSSpy=m_=K3Nov4zaceUX9puQf1TjkTJLA2XC_g@mail.gmail.com [5] https://lore.kernel.org/r/YwIW+mVeZoTOxn%2F4@gmail.com Link: https://lkml.kernel.org/r/20220920122302.99195-2-david@redhat.com Signed-off-by: David Hildenbrand Cc: Andy Whitcroft Cc: Baoquan He Cc: Christophe Leroy Cc: Dave Young Cc: David Laight Cc: Dwaipayan Ray Cc: Ingo Molnar Cc: Jani Nikula Cc: Joe Perches Cc: Jonathan Corbet Cc: Lukas Bulwahn Cc: Michael Ellerman Cc: Nicholas Piggin Cc: Vivek Goyal Signed-off-by: Andrew Morton --- --- a/Documentation/process/coding-style.rst~coding-stylerst-document-bug-and-warn-rules-do-not-crash-the-kernel +++ a/Documentation/process/coding-style.rst @@ -1186,6 +1186,67 @@ expression used. For instance: #endif /* CONFIG_SOMETHING */ +22) Do not crash the kernel +--------------------------- + +In general, it is not the kernel developer's decision to crash the kernel. + +Avoid panic() +============= + +panic() should be used with care and primarily only during system boot. +panic() is, for example, acceptable when running out of memory during boot and +not being able to continue. + +Use WARN() rather than BUG() +============================ + +Do not add new code that uses any of the BUG() variants, such as BUG(), +BUG_ON(), or VM_BUG_ON(). Instead, use a WARN*() variant, preferably +WARN_ON_ONCE(), and possibly with recovery code. Recovery code is not +required if there is no reasonable way to at least partially recover. + +"I'm too lazy to do error handling" is not an excuse for using BUG(). Major +internal corruptions with no way of continuing may still use BUG(), but need +good justification. + +Use WARN_ON_ONCE() rather than WARN() or WARN_ON() +************************************************** + +WARN_ON_ONCE() is generally preferred over WARN() or WARN_ON(), because it +is common for a given warning condition, if it occurs at all, to occur +multiple times. This can fill up and wrap the kernel log, and can even slow +the system enough that the excessive logging turns into its own, additional +problem. + +Do not WARN lightly +******************* + +WARN*() is intended for unexpected, this-should-never-happen situations. +WARN*() macros are not to be used for anything that is expected to happen +during normal operation. These are not pre- or post-condition asserts, for +example. Again: WARN*() must not be used for a condition that is expected +to trigger easily, for example, by user space actions. pr_warn_once() is a +possible alternative, if you need to notify the user of a problem. + +Do not worry about panic_on_warn users +************************************** + +A few more words about panic_on_warn: Remember that ``panic_on_warn`` is an +available kernel option, and that many users set this option. This is why +there is a "Do not WARN lightly" writeup, above. However, the existence of +panic_on_warn users is not a valid reason to avoid the judicious use +WARN*(). That is because, whoever enables panic_on_warn has explicitly +asked the kernel to crash if a WARN*() fires, and such users must be +prepared to deal with the consequences of a system that is somewhat more +likely to crash. + +Use BUILD_BUG_ON() for compile-time assertions +********************************************** + +The use of BUILD_BUG_ON() is acceptable and encouraged, because it is a +compile-time assertion that has no effect at runtime. + Appendix I) References ---------------------- _ Patches currently in -mm which might be from david@redhat.com are mm-gup-replace-foll_numa-by-gup_can_follow_protnone.patch mm-gup-use-gup_can_follow_protnone-also-in-gup-fast.patch mm-fixup-documentation-regarding-pte_numa-and-prot_numa.patch checkpatch-warn-on-usage-of-vm_bug_on-and-other-bug-variants.patch